1. Field of the Invention
This invention relates to the field of elongated flexible structures having longitudinally operable tension or extension mechanisms spaced laterally of a central axis such that the flexible structures can be moved into a curved configuration by applying relatively more longitudinal tension or extension on one side of the axis than the other. More particularly, the invention concerns such a structure wherein a plurality of rigid segments, which are preferably coupled by closed helical springs that space flanged ends of the segments, are provided with control lines for exerting tension between a proximal end of the structure and at least one segment spaced from the proximal end.
2. Prior Art
Controllably bendable resilient structures are known, with segments coupled to define a longitudinal extension, and control lines passing through the segments at points spaced laterally of a central axis. An example is a toy snake which can be curved by shortening one of three laterally spaced control lines as disclosed in U.S. Pat. No. 2,241,576--Barton. The segments must be structured or connected to allow adjacent segments to tilt relative to one another along the axis. In Barton the segments have convex end surfaces which rest against one another at a point. The control lines extend freely through the segments from manually engageable finger rings (at the tail of the snake). The control lines can be pulled through the segments relative to their terminus at the last segment (the head) remote from the rings. By exerting unequal tension on the three laterally spaced control lines it is possible to cause the snake to bend, e.g., to rear its head. In so doing, the point at which adjacent segments contact one another moves laterally toward the inside of the curve. The segments are not connected mechanically to one another except by virtue of being strung like beads on the control lines.
Variations of the idea of curved segments and lateral control lines are disclosed, for example, in U.S. Pat. Nos. 4,393,728 and 4,494,417--both to Larson et al, in connection with a robot painting arm. U.S. Pat. No. 3,266,059--Stelle discloses a similar curved abutting surface in an articulated elbow joint between rigid members of a robot arm.
A variably flexible tether is disclosed in U.S. Pat. No. 3,546,961--Marton. A control cable passes through adjacent segments having concave and convex abutting surfaces. When tension is applied, the segments are pulled against one another and the device becomes relatively more rigid. When tension is released the device is flaccid. The convex/concave abutment between adjacent segments defines the degree of freedom of bending between the segments.
With non-compressible segments abutting at curved surfaces, such devices curve by tension on a control line at the lateral inside of the curve but do not become foreshortened longitudinally. This is because facing parts of the adjacent non-compressible segments remain in contact. The extent of possible bending is defined by the particular structure of the adjacent segments, i.e., by the extent of tilting available until portions of the segments spaced transversely from the longitudinal axis come into contact on the inside of the curve.
Arrangements which have compressible segments or a compressible element between non-compressible segments are relatively foreshortened when tension is applied and elongated when tension is released. Examples are shown in U.S. Pat. Nos. 3,060,972--Sheldon and 4,551,061--Olenick. In U.S. Pat. No. 4,712,969--Kimura, an extensible-retractable arm is provided wherein individually driven expansion-contraction elements are provided between each of the segments.
An important objective in a robot arm or similar controllable appendage is to accurately control the position of the distal end. A welding tool, spray head, grasping apparatus, video camera or any of various other structures can be mounted on the arm, and oriented or manipulated (e.g., applied to a workpiece) in a programmed manner. However, an arm comprising non-compressible segments with curved abutting faces is difficult to control accurately and to keep suitably stiff because there is no real connection between the adjacent segments. On the other hand, the longitudinal expansion and contraction inherent in resiliently coupled segments, which varies as tension is applied or changed to achieve a particular curve, makes accurate position programming difficult or impossible.
In U.S. Pat. Nos. 3,497,083--Anderson et al and 4,566,843--Iwatsuka et al, segments are coupled by universal joints between adjacent segments, defined by pivot axes oriented at right angles. These joints are non-compressible, but are heavy, complicated and expensive. Additionally, the joint structures eliminate the potential of an open lumen along the central axis of the arm, for passage of fluid lines and/or electrical lines, or at least substantially occlude the available space for such lines.
Assuming the flexible structure is applied to a device for positioning a free distal end, for example carrying a tool, the load to be borne by segments disposed closer to the proximal end is greater than the load for segments at the distal end because the proximal segments must carry the weight of the distal segments. Anderson and Iwatsuka use progressively smaller segments or groups of segments proceeding toward the distal end. Each also provides separate control lines for the different segments or groups. The control lines for the larger, proximal segments are more laterally spaced than those for the smaller, distal segments, which enables greater leverage to be applied to generate a proximal curve.
In Anderson and Iwatsuka the maximum limit of the curve available between adjacent segments (i.e., the minimum radius of curvature) is reached when the segments abut one another at contact points located at a lateral space from the center of the universal joint along the longitudinal axis. The universal joint defines a secure, if heavy, means for fixing the alignment of adjacent segments. This security of alignment, however, has the unfavorable result that when first applying tension to a control line, only the endmost segment attached to the control line, generally the closest proximal segment, becomes tilted relative to the next adjacent segment. This adjacent segment is not urged to tilt until the more proximal segment curves to reach the contact point defining its limit. With increasing tension, the curve of the arm as a whole begins with tilting of segments exclusively at the base or proximal area, and proceeds outwardly, segment by segment, as each of the segments in turn is curved to its limit.
It would be advantageous to provide a flexible positioning appendage which has good flexibility and bends in a continuous manner along its length, but which has sufficient structural integrity to support itself, plus a load on the free or distal end. The appendage should be controllably bendable at any area (or even individual joint) along its length, independent of curves at other areas. Preferably, this should be achieved in a device which defines a passageway for fluid or electrical lines and the like, and does not necessarily contract or elongate when tension is applied and released. Generally, it is desirable to provide for progressively greater stiffness and support proceeding towards the proximal end, however, a given application may require the capability of a pronounced bend at a certain point along the appendage. This is provided according to the invention by certain arrangements of resilient couplings and resilient segments, facilitating an appropriate selection of rigidity vs. flexibility which is apt for a number of applications as discussed in detail hereinafter.